The present invention relates to a process for producing molten iron from a metalliferous feed material, such as ores, partly reduced ores, and metal-containing waste streams, in a molten bath-based direct smelting process for producing molten iron from a metalliferous feed material.
The term xe2x80x9cdirect smelting processxe2x80x9d is understood to mean a process that produces a molten material, in this case iron, from a metalliferous feed material.
One known molten bath-based direct smelting process for producing molten ferrous metal is the DIOS process. The DIOS process includes a pre-reduction stage and a smelt reduction stage. In the DIOS process ore (xe2x88x928 mm) is pre-heated (750xc2x0 C.) and pre-reduced (10 to 30%) in bubbling fluidised beds using offgas from a smelt reduction vessel which contains a molten bath of metal and slag, with the slag forming a deep layer on the metal. The fine (xe2x88x920.3 mm) and coarse (xe2x88x928 mm) components of the ore are separated in the pre-reduction stage of the process and the xe2x88x920.3 mm is collected in a cyclone and injected into the smelt reduction furnace with nitrogen whilst the coarse ore is charged by gravity. Pre-dried coal is charged directly to the smelt reduction furnace from the top of the vessel. The coal decomposes into char and volatile matter in the slag layer and the ore dissolves in the molten slag and forms FeO. The FeO is reduced at the slag/metal and slag/char interfaces to produce iron. The carbon monoxide generated at the metal/slag and slag/char interface generates a foaming slag. Oxygen is blown through a specially designed lance that introduces the oxygen inside the foamed slag and improves secondary combustion. Oxygen jets burn carbon monoxide that is generated with the smelting reduction reactions, thereby generating heat that is transferred first to the molten slag and then to the slag/metal interface by the strong stirring effect of bottom blowing gas. The stirring gas introduced into the hot metal bath from the bottom or side of the smelt reduction vessel improves heat transfer efficiency and increases the slag/metal interface for reduction and therefore the vessel productivity and thermal efficiency. However, injection rates must be limited as strong stirring lowers secondary combustion due to increased interaction between the oxygen jet and metal droplets in the slag with subsequent lowering of productivity and increased refractory wear. Slag and metal are tapped periodically.
Another known direct smelting process for producing molten ferrous metal is the Romelt process. The Romelt process is based on the use of a large volume, highly agitated slag bath as the medium for smelting metalliferous feed material to metal in a smelt reduction vessel and for post-combusting gaseous reaction products and transferring the heat as required to continue smelting metalliferous feed material. The metalliferous feed material, coal, and fluxes are gravity fed into the slag bath via an opening in the roof of the vessel. The Romelt process includes injecting a primary blast of oxygen-enriched air into the slag via a lower row of tuyeres to cause necessary slag agitation and injection of oxygen-enriched air or oxygen into the slag via an upper row of tuyeres to promote post-combustion. The molten metal produced in the slag moves downwardly and forms a metal layer and is discharged via a forehearth. In the Romelt process the metal layer is not an important reaction medium.
Another known direct smelting process for producing molten ferrous metal is the AISI process. The AISI process includes a pre-reduction stage and a smelt reduction stage. In the AISI process pre-heated and partially pre-reduced iron ore pellets, coal or coke breeze and fluxes are top charged into a pressurised smelt reactor which contains a molten bath of metal and slag. The coal devolatilises in the slag layer and the iron ore pellets dissolve in the slag and then are reduced by carbon (char) in the slag. The process conditions result in slag foaming. Carbon monoxide and hydrogen generated in the process are post combusted in or just above the slag layer to provide the energy required for the endothermic reduction reactions. Oxygen is top blown through a central, water cooled lance and nitrogen is injected through tuyeres at the bottom of the reactor to ensure sufficient stirring to facilitate heat transfer of the post combustion energy to the bath. The process offgas is de-dusted in a hot cyclone before being fed to a shaft type furnace for pre-heating and pre-reduction of the pellets to FeO or wustite.
Another known direct smelting process which, unlike the above-described processes, relies on a molten metal layer as a reaction medium is generally referred to as the HIsmelt process and includes the steps of:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in a direct smelting vessel;
(b) injecting metalliferous feed material and coal into the metal layer via a plurality of lances/tuyeres;
(c) smelting metalliferous material to metal in the metal layer;
(d) causing molten material to be projected as splashes, droplets, and streams above a quiescent surface of the molten bath to form a transition zone; and
(d) injecting an oxygen-containing gas into the vessel via one or more than one lance/tuyere to post-combust reaction gases released from the molten bath, whereby ascending and thereafter descending splashes, droplets and streams of molten material in the transition zone facilitate heat transfer to the molten bath, and whereby the transition zone minimises heat loss from the vessel via the side walls in contact with the transition zone.
A preferred form of the HIsmelt process is characterized by forming the transition zone by injecting carrier gas, metalliferous feed material, coal, and fluxes into the bath through lances that extend downwardly and inwardly through side walls of the vessel so that the carrier gas and the solid material penetrate the metal layer and cause molten material to be projected from the bath.
This form of the HIsmelt process is an improvement over earlier forms of the process which form the transition zone by bottom injection of carrier gas and coal through tuyeres into the bath which cause droplets and splashes and streams of molten material to be projected from the bath.
The Romelt, DIOS, AISI and HIsmelt direct smelting processes can use coal as the source of energy and reductant. This is an advantage of the direct smelting processes over blast furnace technology which requires coke as the source of energy/reductant.
The Romelt, DIOS, AISI and HIsmelt direct smelting processes can operate with a wide range of metalliferous feed materials.
Iron ore is the major source of metalliferous feed materials for producing molten iron via the Romelt, DIOS, AISI, and HIsmelt processes.
One process option for the direct smelting processes is to supply iron ore directly to direct smelting vessels.
Another process option is to pre-heat and partially reduce iron ore in a solid state in pre-reduction vessels (which could be a shaft furnace or a fluidised bed or any other suitable vessel), transfer the pre-heated/partially reduced iron ore to direct smelting vessels containing a molten bath of iron and slag, and smelt the pre-heated/partially reduced iron ore to molten iron in the direct smelting vessels. This process option may also include using off-gas from the direct smelting vessels to pre-heat/pre-reduce iron ore in the pre-reduction vessels. One advantage of the process option is that it provides an opportunity to minimise total energy consumption. One disadvantage of the process option is that undesirable impurities, typically coal-derived impurities such as sulphur and alkali salts, which volatilise in direct smelting vessels and are discharged as part of the off-gas, return to the direct smelting vessels with the pre-heated/partially reduced iron ore and accumulate in the vessels. Specifically, sulphur reacts with FeO in the pre-reduction vessels and forms FeS and alkali salts condense in the pre-reduction vessels, and the FeS and condensed alkali salts are transferred to the direct smelting vessels with the pre-heated/partially reduced iron ore. The return of FeS into a direct smelting vessel disrupts the reaction sites of the smelting process and can significantly affect production. One solution to this issue is to increase the temperature of the medium for smelting. However, this leads to increased refractory wear and if pursued too far leads to the partitioning of phosphorus into the metal rather than the slag, and this is a major disadvantage.
An object of the present invention is to alleviate the disadvantage of the known 2-stage direct smelting process described in the preceding paragraph and in particular where the smelting medium is metal.
According to the present invention there is provided a process for direct smelting metalliferous feed material which includes the steps of:
(a) partially reducing iron oxides in a solid state in a pre-reduction vessel and producing partially reduced iron oxides;
(b) direct smelting partially reduced iron oxides produced in step (a) to molten iron in a direct smelting vessel which contains a molten bath of iron and slag and is supplied with a solid carbonaceous material as a source of reductant and energy and with an oxygen-containing gas for post-combusting carbon monoxide and hydrogen generated in the vessel;
(c) generating an off-gas that contains sulphur in direct smelting step (b) and releasing the off-gas from the direct smelting vessel; and
(d) using only part of the off-gas released from the direct smelting vessel in pre-reduction step (a) to pre-reduce iron oxides in the pre-reduction vessel to control the amount of sulphur that is returned to the direct smelting vessel from the pre-reduction vessel.
The effect of step (d) of using only part rather than all of the off-gas from the direct smelting vessel in pre-reduction step (a) is to at least minimise the rate of build-up of undesirable impurities, typically coal-derived impurities, in the direct smelting vessel. As is indicated above, a disadvantage of the known 2-stage direct smelting process is that a number of undesirable impurities, typically coal-derived impurities such as sulphur and alkali salts, that are volatilised in direct smelting vessels are recovered in pre-reduction vessels and thereafter are returned to the direct smelting vessels.
Preferably step (d) includes controlling the amount of off-gas released from the direct smelting vessel and used in pre-reduction step (a) so that the amount of sulphur in molten iron produced in direct smelting step (b) is less than 0.2 wt % of the total weight of the molten iron.
Preferably the process includes processing the remainder of the off-gas released from the direct smelting vessel for heating and/or for power generation without returning the majority of the sulphur in this part of the off-gas to the direct smelting vessel.
Preferably direct smelting step (b) includes injecting pre-heated air or oxygen-enriched air into the direct smelting vessel as the oxygen-containing gas.
More preferably the process includes using a first stream of the off-gas from the direct smelting vessel in pre-reduction step (a) and using a second stream of the off-gas as a source of energy for heating air or oxygen-enriched air before supplying the air or oxygen-enriched air to the direct smelting vessel.
Preferably the second stream includes at least 20% by volume of the off-gas released from the direct smelting vessel.
More preferably the second stream includes at least 30 vol. % of the off-gas released from the direct smelting vessel.
It is preferred particularly that the second stream includes at least 40 vol. % of the off-gas released from the direct smelting vessel.
Preferably the process includes removing entrained sulphur and alkali salts from the second stream prior to using the second stream as the source of energy for heating air or oxygen-enriched air.
Preferably the oxygen-enriched air contains less than 50 volume % oxygen.
Preferably pre-reduction step (a) pre-heats the iron ore to a temperature in the range of 600-1000xc2x0 C.
Preferably the off-gas from pre-reduction step (a) is used as a fuel gas for heating or power generation.
Smelting step (b) may include any suitable direct smelting process and use either the metal or the slag as the smelting medium.
Preferably smelting step (b) includes using the metal as a smelting medium and more preferably as the principal smelting medium.
Preferably smelting step (b) includes direct smelting partially reduced iron oxides in accordance with the HIsmelt process which includes the steps of:
(i) forming the molten bath with a molten iron layer and a molten slag layer on the iron layer in the direct smelting vessel;
(ii) injecting the partially reduced iron oxides and coal into the iron layer via a plurality of lances/tuyeres;
(iii) smelting the partially reduced iron oxides to molten iron in the iron layer;
(iv) causing molten material to be projected as splashes, droplets, and streams into a space above a nominal quiescent surface of the molten bath and forming a transition zone; and
(v) injecting the oxygen-containing gas into the direct smelting vessel via one or more than one lance/tuyere and post-combusting carbon monoxide and hydrogen released from the molten bath, whereby the ascending and thereafter descending splashes, droplets, and streams of molten material in the transition zone facilitate heat transfer to the molten bath, and whereby the transition zone minimises heat loss from the vessel via a side wall of the vessel that is in contact with the transition zone.
The term xe2x80x9cquiescent surfacexe2x80x9d in the context of the molten bath is understood herein to mean the surface of the molten bath under process conditions in which there is no gas/solids injection and therefore no bath agitation.